Tens of thousands of Thomas Cook customers applied for refunds on the first day of a claims website opening.
More than 60,000 claims were made on Monday by people whose holidays were cancelled, the Civil Aviation Authority (CAA) said.
The regulator was forced to apologise after its system struggled to cope with “unprecedented demand” in the hours after it launched at 6am.
Many people logging on received an error message after entering their details, meaning their claims were not submitted.
Up to 200 claims per minute were made on Monday.
Twitter user @kevharrison_ posted a message to the CAA which read: “What the hell are you doing? You had weeks to make sure this system is robust.”
Another frustrated customer, with the user name @todd_alexandra, described the claims portal as “pointless and frustrating” before urging the regulator to “address this asap”.
The website was created to enable passengers with Atol-protected bookings for Thomas Cook holidays due to begin after the firm collapsed on September 23 to claim refunds.
This relates to more than 360,000 bookings covering trips due to be taken by 800,000 people.
Atol-protected customers who were already abroad when Thomas Cook failed can also claim for the cost of replacing the parts of their holiday which were financially protected, or out-of-pocket expenses for delayed flights.
The CAA is aiming to pay refunds within 60 days of receiving a valid form.
Adam French, of consumer group Which?, said: “It is understandably frustrating for some Thomas Cook customers that they are now facing problems trying to submit their refunds because of high volumes of online traffic.
“It is vital that customers persevere and do not turn to alternative websites, as we’ve seen scammers seeking to take advantage of this situation by setting up convincing fake refund websites to fleece customers.”
The CAA’s final repatriation flight arrived at Manchester Airport from Orlando on Monday morning.
About 140,000 Thomas Cook passengers were brought home from around the world on 150 aircraft during the past two weeks.
The vast majority of people flew on their original travel date.
CAA chief executive Richard Moriarty said: “The largest peacetime repatriation ever required an extraordinary effort from all involved.
“I want to thank everyone who has played their part in delivering this enormous undertaking, including the passengers we flew home for bearing with us as we undertook this complex operation.
“I also want to pay tribute to the many amazing former Thomas Cook employees who worked with us to make this operation a success.
“It needed an unprecedented team effort from our commercial partners, our friends across Government and my colleagues at the CAA.”
– Affected consumers can find out more information at https://thomascook.caa.co.uk/